Effective O2 diffusion from the red blood cell to the muscle fiber is necessary to sustain cellular integrity and support oxidative function. There is increasing evidence that the principal resistance to O2 diffusion in skeletal muscle is extracellular. Under various physiological and pathological conditions, O2 diffusion is impaired resulting in muscle fatigue/failure and sometimes degenerative changes. Morphometric techniques have described the consequences of acute and chronic perturbations on capillary geometry. However, the effects of these changes on RBC hemodynamics, O2 exchange and muscle function have not been determined. The objective of the proposed investigations is to utilize established models of pathophysiological conditions to reconfigure capillary geometry and determine the relationships between capillary geometry, capillary hemodynamics, O2 exchange and muscle function. Specifically, it is hypothesized that capillary adaptations (reduced luminal diameter, involution, enhanced stretch) found in skeletal muscle of animals with diabetes, or those suffering from chronic heart failure, will act to impair RBC flux and augment RBC distribution heterogeneity thereby reducing blood-tissue O2 exchange and muscle performance. Two muscles of similar fiber type but differing in oxidative capacity (spinotrapezius and diaphragm) will be utilized. Experimental conditions are designed to stress the capacity for muscle blood flow and O2 exchange (vasodilation, hypoxemia/hyperoxemia, muscle contractions). Local vasodilation will be used to distinguish between arteriolar vasoregulatory effects and those arising from capillary geometrical alterations. Microvascular and muscle function will be assessed using intravital microscopy, phosphorescence quenching (microvascular PO2) and muscle tension measurements. These studies will combine established optical, phosphorescence and functional technologies to investigate the capacity for capillary geometrical changes to perturb muscle O2 exchange to obtain information regarding the role of capillary geometry in facilitating muscle O2 delivery in health and a series of common disease states.